Method of film-forming of tungsten

ABSTRACT

A method of forming a tungsten film is capable of forming a tungsten film having a low resistivity. The method of forming a tungsten film (50) on a surface of a workpiece by a vacuum processing system (2) comprises, in sequential steps: a seed crystal growing process for growing tungsten seed crystal grains (48) on the surface of the workpiece in an atmosphere of a film forming gas containing tungsten atoms; a boron-exposure process for exposing the workpiece to an atmosphere of a boron-containing gas for a short time; and a tungsten film forming process for forming a tungsten film by making the tungsten seed crystal grains grow in an atmosphere of a gas containing a film forming gas containing tungsten atoms, and a hydrogen-diluted boron-containing gas. The tungsten film has a low resistivity.

TECHNICAL FIELD

The present invention relates to a method of forming a tungsten filmhaving an improved resistivity.

BACKGROUND ART

Generally, a film of a metal or a metal compound, such as W (tungsten),WSi (tungsten silicide), Ti (titanium), TiN (titanium nitride)or TiSi(titanium silicide), is deposited to form a wiring pattern on asemiconductor wafer, to fill up holes between wiring lines or to formwiring patterns and fill up holes between wiring lines in asemiconductor integrated circuit fabricating process.

Methods of forming such metal thin films are classified into those of H₂reduction system (hydrogen reduction system), those of SiH₄ reductionsystem (Silane reduction system) and those of SiH₂Cl₂ reduction system(dichlorosilane reduction system). When forming a wiring pattern by themethod of SiH₂Cl₂ reduction system, dichlorosilane gas is used as areducing gas and a W or a WSi film (tungsten silicide film) is formed ata high temperature on the order of 600° C. When forming a wiring patternby the method of SiH₄ reduction system, silane gas is used as a reducinggas and a W or WSi film is formed at a low temperature on the order of350° C.

The method of H₂ reduction system is applied mainly to filling up holesin the surface of a wafer, such as holes between wiring lines, useshydrogen gas as a reducing gas and deposits a W film at a temperature inthe range of about 400° to about 430° C.

All those methods use, for example, WF₆ (tungsten hexaf luoride). Aconventional tungsten film forming method will be explained. A thinTi/TiN film, for instance, is formed as a barrier metal layer on asurface of a semiconductor wafer before forming a tungsten film. Filmforming gases including WF₆ gas, SiH₄ gas, H₂ gas, Ar gas, N₂ gas andthe like are supplied into a film forming chamber to deposit tungstenseed crystal grains on the surface of the barrier metal layer.

The film forming chamber is evacuated temporarily to a base pressure toremove residual gases from the film forming chamber, and then the Argas, H₂ gas and N₂ gas are supplied into the film forming chamber to setthe film forming chamber quickly at a process pressure. Subsequently,WF₆ gas is supplied at a predetermined flow rate into the film formingchamber to deposit a tungsten film by hydrogen reduction using H₂ gaswithout using SiH₄ gas. Thus, for example, filling up holes and forminga wiring layer are carried out simultaneously.

The development of multilayer semiconductor integrated circuits, and theprogressive miniaturization and rise in the level of integration requirefurther reduction of width of lines and diameters of holes. When awiring pattern is miniaturized, the resistance of wiring lines increasesaccordingly. Resistivity of wiring lines low enough for conventionaldesign must be reduced further when wiring patterns are miniaturized.

However, it has been difficult to form tungsten films having asatisfactorily low resistivity and meeting new design by the foregoingconventional tungsten film forming method.

A method intended to form a tungsten film having a reduced resistivityto solve the foregoing problems supplies a bo-ron-containing gas such asdiborane (B₂H₆) gas with Ar gas and N₂ gas into the film forming chamberto form a tungsten film of tungsten crystal grains of large grain sizesto reduce resistivity. This method, however, is incapable of forming thetungsten film having satisfactorily low resistivity, and thenitrogen-diluted borane gas produces a solid by polymerization in gassupply pipes and the solid clogs the gas supply pipes.

Generally, unnecessary films are deposited in the processing vessel of afilm forming system as a film forming process is repeated certaincycles, and the films fall off in particles. Therefore, a cleaningprocess is carried out at regular or irregular intervals to remove theunnecessary films by supplying a cleaning gas, such as ClF₃ gas, intothe processing vessel. The cleaning gas remains, though in only a verysmall amount, in the processing vessel after cleaning and Cl and F atomscontained in ClF₃ gas are introduced into the surface of a semiconductorwafer and act as detrimental impurities.

The present invention has been made in view of such problems and hasbeen created to solve those problems effectively. It is therefore anobject of the present invention to provide a tungsten film formingmethod capable of forming a tungsten film having a low resistivity.

DISCLOSURE OF THE INVENTION

The inventors of the present invention made earnest studies of tungstenfilm forming methods and acquired a knowledge that crystal grains oflarge grain sizes can be formed by using a gas containing hydrogen and aborane, such as diborane, for forming a tungsten film, and crystalgrains of large grain sizes can be formed by carrying out a tungstenfilm forming process immediately after processing a semiconductor waferby a boronizing surface treatment process using a boron-containing gas,such as B₂H₆ gas, and have made the present invention.

It is a first feature of the present invention that a method of forminga tungsten film on a surface of an object to be processed by a vacuumprocessing system, said method comprising the steps of: growing tungstenseed crystal grains on the surface of the object to be processed in anatmosphere of a film forming gas containing tungsten atoms; exposing theobject to be processed to an atmosphere of a boron-containing gas for ashort time; and forming a tungsten film by making the tungsten seedcrystal grains grow in an atmosphere of a gas containing a film forminggas containing tungsten atoms, a hydrogen gas and a hydrogen-dilutedboron-containing gas.

When forming the tungsten film after growing seed crystal grains oftungsten on the workpiece and exposing the workpiece to the atmosphereof the boron-containing gas for a short time, the film forming gas issupplied to the workpiece in the presence of boron-containing gas andhydrogen gas. Therefore, tungsten crystal grains forming the tungstenfilm grow large and thereby the tungsten film having a low resistivitycan be formed.

It is a second feature of the present invention that a method of forminga tungsten film on a surface of an object to be processed by a vacuumprocessing system, said method comprising the steps of: growing tungstenseed crystal grains on the surface of the object to be processed in anatmosphere of a film forming gas containing tungsten atoms; and forminga tungsten film by making the tungsten seed crystal grains grow in anatmosphere of a gas containing a film forming gas containing tungstenatoms, hydrogen gas, and a hydrogen-diluted boron-containing gas.

Although the tungsten film forming method according to the second aspectof the present invention omits the boron-exposure process included inthe tungsten film forming method according to the first aspect of thepresent invention, the same method is able to form a tungsten filmhaving a relatively low resistivity.

It is a third feature of the present invention that, when theboron-containing gas is a 5% hydrogen-diluted B₂H₆gas, the flow rate ofthe boron-containing gas is about 0.85% or above of the total flow rateof all the gases. When the boron-containing gas is supplied at such aflow rate, the resistivity is included in a preferable range, theboron-containing gas undergoes self decomposition and boron adsorptionand bonding occur on a growth surface in the boron-exposure process.

It is a fourth feature of the present invention that a seed crystalgrain layer formed in the seed crystal growing process is 50 nm or belowin thickness. The seed crystal grain layer of such a thickness ispreferable in view of forming a tungsten film having a low resistivity.

It is a fifth feature of the present invention that the total amount ofall the gases supplied every minute in the tungsten film forming processis about 100% or above of the volume of the processing vessel of thevacuum processing system. Supply of the gases at such a flow rate ispreferable in view of forming a tungsten film having a low resistivity

It is a sixth feature of the present invention that, the tungsten filmforming process achieves simultaneously, for example, both filling upholes formed in the surface of the object to be processed and formingwiring lines.

It is a seventh feature of the present invention that method of forminga tungsten film on a surface of an object to be processed by a vacuumprocessing system, said method comprising the steps of: growing tungstenseed crystal grains on the surface of the object to be processed in anatmosphere of a film forming gas containing tungsten atoms; exposing theobject to be processed to an atmosphere of a boron-containing gas for ashort time; and forming a tungsten film by making the tungsten seedcrystal grains grow in an atmosphere of a gas containing a film forminggas containing tungsten atoms.

When the tungsten seed crystal grains are processed by boron surfacetreatment to expose the workpiece to an atmosphere of a boron-containinggas after the formation of the tungsten seed crystal grains andimmediately before the formation of the tungsten film, a tungsten filmof large tungsten crystal grains having a low resistivity can be formed.

It is an eighth feature of the present invention that, when theboron-containing gas is a 5% hydrogen-diluted B₂H₆ gas, the flow rate ofthe boron-containing gas is about 0.85% or above of the total flow rateof all the gases. When the boron-containing gas is supplied at such aflow rate, a tungsten film having a considerably low resistivity can beformed.

It is a ninth feature of the present invention that the tungsten filmforming process achieves simultaneously, for example, both filling upholes formed in the surface of the object to be processed and formingwiring lines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a vacuum processing system forcarrying out a tungsten film forming method according to the presentinvention;

FIGS. 2A, 2B, 2C and 2D are fragmentary, typical sectional views ofassistance in explaining a preparatory process, a seed crystal growingprocess, a boron-exposure process and a tungsten film forming process ofa film forming method in a first embodiment according to the presentinvention, respectively;

FIG. 3 is a graph comparatively showing the resistivity of a film formedby the tungsten film forming method of the present invention and that ofa film formed by a conventional tungsten film forming method not usingB₂H₆;

FIG. 4 is a graph showing the dependence of resistivity on the flow rateof 5% B₂H₆ gas;

FIG. 5 is a graph showing the dependence of resistivity on 5% B₂H₆ gassupply rate;

FIG. 6 is a graph showing the dependence of resistivity on the totalflow rate of the gases;

FIG. 7 is a graph showing the dependence of resistivity on the thicknessof a tungsten seed crystal grain layer;

FIG. 8 is a graph showing the variation of diborane (B₂H₆) concentrationwith time for a case when a nitrogen-diluted B₂H₆ gas is used(conventional method) and a case when a hydrogen-diluted B₂H₆ gas isused (present invention);

FIGS. 9A, 9B, 9C and 9D are fragmentary, typical sectional views ofassistance in explaining a preparatory process, a seed crystal growingprocess, a pressure raising process, and a tungsten film forming processof a film forming method in a second embodiment according to the presentinvention, respectively;

FIG. 10 is a graph showing sectional profiles of boron concentration;

FIG. 11 is a graph showing sectional profiles of chlorine concentration;

FIG. 12 is a graph showing sectional profiles of fluorine concentration;and

FIGS. 13A and 13B are photographs taken by an electron microscope ofsections of holes filled up with a tungsten film by a conventionalmethod and a method according to the present invention, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

A tungsten film forming method in a preferred embodiment according tothe present invention will be described hereinafter with reference tothe accompanying drawings.

FIG. 1 is a schematic view of a vacuum processing system for carryingout the method according to the present invention.

The vacuum processing system for carrying out the method according tothe present invention will be described.

The vacuum processing system 2 is provided with a cylindrical processingvessel 4 of aluminum or the like. A cylindrical reflector 6 is set in anupright position on the bottom of the processing vessel 4. A stage 10for supporting a semiconductor wafer W, i.e., a workpiece, thereon issupported by L-shaped holding members 8 on the reflector 6. The stage 10is, for example, is formed of carbon or an aluminum compound, such asAlN, and has a thickness on the order of several millimeters.

A transparent window 12 of quartz is set directly below the stage 10 andis attached hermetically to the bottom wall of the processing vessel 4.A box-shaped heating vessel 14 is disposed under the transparent window12 so as to surround the latter. A plurality of heating lamps 16attached to a rotary table 18 serving also as a reflector is disposed inthe heating vessel 14. The rotary table 18 is driven for rotation by amotor 20. Heat rays radiated by the heating lamps 16 travel through thetransparent window 12 and fall on the lower surface of the stage 10 toheat the wafer W supported on the stage 10.

Exhaust ports 22 are formed in peripheral portions of the bottom wall ofthe processing vessel 4, and exhaust pipes 24 connected to a vacuum pumpare connected to the exhaust ports 22 to evacuate the interior of theprocessing vessel 4. A gate valve 26 is attached to the side wall of theprocessing vessel 4. The wafer is carried into the processing vessel 4through the gate valve 26.

A shower head 28 for distributing process gases into the processingvessel 4 is attached to the top wall of the processing vessel 4 oppositeto the stage 10. The shower head 28 has a spouting wall 28A providedwith a plurality of gas spouting holes 30, and a top wall provided witha gas inlet port 32. A gas supply system for supplying necessary gasesfor film forming and the like is connected to the gas inlet port 32 ofthe shower head 28.

Concretely, a boron-containing gas for a tungsten seed crystal formingprocess can be supplied to the shower head 28. Process gas sources forsupplying WF₆ gas, Ar gas, SiH₄ gas, H₂ gas, N₂ gas and B₂H, gas areconnected to the shower head 28. Each of pipes connecting the processgas sources to the shower head 28 is provided with a mass-flowcontroller 34, i.e., flow controller, and two shutoff valves 36 and 38disposed on the opposite sides of the mass-flow controller 34,respectively. The flow rate of each gas can be controlled and the gascan selectively be supplied or stopped.

Diborane gas (B₂H₆ gas) is used as the boron-containing gas. Theboron-containing gas is not 100% B₂H₆ gas; The boron-containing gas is a5% hydrogen-diluted B₂H₆ gas.

The volume of the processing vessel 4 is about 1200 cm³. The stage 10has a diameter of about 200 mm and is capable of supporting an 8 in.wafer thereon.

A tungsten film forming method in a first embodiment (example 1)according to the present invention to be carried out by the vacuumprocessing system thus constructed will be described with reference toFIG. 2.

The gate valve 26 attached to the side wall of the processing vessel 4is opened, a wafer W is carried into the processing vessel 4 and isplaced on the stage 10 by a carrying arm, not shown. For example, a thinbarrier metal layer 40 of Ti/TiN is formed on a surface of the wafer Was shown in FIG. 2A by a preparatory process. The barrier metal layer 40is formed so as to cover the inner surfaces of holes 42, such as contactholes and via holes. The holes 42 have diameters in the range of, forexample, about 0.5 to about 1.0 μm, and aspect ratios in the range ofabout 1 to about 2, respectively. In FIG. 2, indicated at 44 is a dopedpolysilicon film and at 46 is an insulating film.

Process gasses WF₆ gas, SiH₄ gas, H₂ gas, Ar gas and N₂ gas are suppliedat predetermined flow rates, respectively, from the process gas sourcesto the shower head 28, and a mixture of those process gases are spoutedthrough the gas spouting holes 30 substantially uniformly into theprocessing vessel 4. B₂H₆ gas is not supplied at this stage. At the sametime, the interior atmosphere of the processing vessel 4 is evacuatedthrough the exhaust ports 22 to maintain the interior of the processingvessel 4 at a predetermined vacuum on the order of, for example, 4 torr,and the heating lamps 16 are turned and are made to radiate thermalenergy.

Heat rays radiated from the heating lamps 16 travel through the window12 and fall on the back surface of the stage 10 to heat the stage 10.Since the thickness of the stage 10 is as very small as severalmillimeters, the stage 10 is heated rapidly and hence the wafer Wmounted on the stage 10 can be rapidly heated up to a predeterminedtemperature. Process temperature is, for example, about 460° C. Themixed gas supplied into the processing vessel 4 undergoes apredetermined chemical reaction and, as shown in FIG. 2B, WF₆, isreduced and tungsten seed crystal grains 48 are formed on the surface ofthe barrier metal layer 40 for a seed crystal growing process. The seedcrystal growing process is continued, for example, for about 30 s toform an about 30 nm thick seed crystal grain layer.

After the completion of the seed crystal growing process, aboron-exposure process is started.

First, the supply of all the process gases is stopped, the processingvessel 4 is evacuated to a base pressure of, for example, on the orderof 10⁻³ torr, and predetermined gases including B₂H₆ gas is supplied tomaintain the pressure in the processing vessel 4 at about 80 torr for ashort time for the boron-exposure process as shown in FIG. 2C. Ar gas,H₂ gas and a 5% B_(2H) ₆ gas (hydro-gen-diluted) are supplied at 4000,1800 and 100 sccm, respectively. WF₆ gas, SiH₄, gas and N₂ gas are notsupplied. The tungsten seed crystal grains 48 are exposed to boron, B₂H₆gas decomposes, and boratite is formed on the tungsten seed crystalgrain layer. Consequently, the seed crystal grains grow to some extent.The boron-exposure process is continued, for example, for about 28 s atabout 460° C.

After the completion of the boron-exposure process, a tungsten filmforming process is started.

First, WF₆ gas, Ar gas, B2 gas and 5% B₂H₆ gas (hydro-gen-diluted) aresupplied at 25, 4400, 600 and 25 sccm, respectively, to form a tungstenfilm. The supply of SiH₄ gas and N₂ gas is stopped. The same processpressure of 80 torr and the same process temperature of 460° C. as thoseof the preceding process are used. Consequently, holes 42 (FIG. 2A) arefilled up and, at the same time, a wiring tungsten film 50 is formed asshown in FIG. 2D. The duration of the tungsten film forming is, forexample, about 40 s. The overall thickness of the tungsten film 50 is100 nm

The boron-exposure process is carried out subsequently to the seedcrystal growing process to expose the tungsten seed crystal grains todiborane for hydrogen reduction to form boratite partially, and B₂H₆ gasis supplied also in the tungsten film forming process for hydrogenreduction to form boratite while the tungsten film is formed. Therefore,the tungsten seed crystal grains grow considerably. Therefore, thetungsten film 50 has crystal structure similar to bulk crystal structureand has a considerably low resistivity.

Whereas the resistivity of a tungsten film formed by the conventionaltungsten film forming method not using B₂H₆ gas was about 12.2 μΩcm(1500 Å), the resistivity of a tungsten film formed by the tungsten filmforming method according to the present invention was about 8.0 μcm(1500 Å), which proved significant resistivity improvement.

As obvious from FIG. 3 comparatively showing the resistivity of atungsten film formed by the conventional tungsten film forming methodand that of a tungsten film formed by the tungsten film forming methodaccording to the present invention, the resistivity of the tungsten filmformed by the method according to the present invention is lower byabout 40% than that of the tungsten film formed by the conventionalmethod regardless of thickness.

The boron content (B content), the chlorine content (Cl content) andfluorine content (F content) of the wafer including the tungsten filmformed by the method according to the present invention and the waferincluding the tungsten film formed by the conventional method weremeasured. The B content of the wafer processed by the method accordingto the present invention, as a matter of course, was large, and the Cland the F content of the same were considerably small, which proved thatthe detrimental effect of a residual cleaning gas can be suppressed. Theresult of the measurement of this point will be described later.

The method according to the present invention had no problem at all infilling up the holes 42 and the method according to the presentinvention, similarly to the conventional method, could satisfactorilyfill up the holes 42.

The tungsten film forming method in the first embodiment (example 1)according to the present invention uses the hydro-gen-diluted 5% B₂H₆gasat 100 sccm. It is preferable that the flow rate of the hydrogen-diluted5% B₂H₆ gas is about 50 sccm or above, i.e., about 0.85% of the totalflow rate of all the gasses (≈50×100/(4000+180+50)) or above. If theflow rate of the hydrogen-diluted 5% B₂H₆ gas is less than about 50sccm, the resistivity is not significantly low, which will be describedwith reference to FIG. 4. In order to show the results described above,FIG. 4 shows the dependence of resistivity (1800 Å) on the flow rate ofthe hydrogen-diluted 5% B₂H₅ gas. As mentioned above, the respectiveflow rates of Ar gas and H₂ gas are fixed at 4000 sccm and 1800 sccm.

As obvious from the graph shown in FIG. 4, the resistivity of thetungsten film is undesirably high and higher than 11.3 μΩcm when theflow rate of the hydrogen-diluted 5% B₂H₆ gas is below 50 sccm. When theflow rate of the hydrogen-diluted 5% B₂H₆, gas is not lower than 50sccm, the resistivity is less than 11 μΩcm. Thus, the measurements showsthat it is desirable to supply the 5% B₂H₆ gas in the boron-exposureprocess at a flow rate of 50 sccm or above (0.85% or above of the totalflow rate of all the gases).

In the tungsten film forming process, differently from in theboron-exposure process, if the hydrogen-diluted 5% B₂H₆ gas is suppliedat an excessively high rate, the resistivity increases unpreferably. Forexample, it is preferable that the flow rate of the hydrogen-diluted 5%B₂H₆ gas is about 2% or below of the total flow rate of all the gases.The lower limit of percentage of the flow rate of the hydrogen-diluted5% B₂H₆ gas to the total flow rate of all the gases is about 0.2%. Ifthe hydrogen-diluted 5% B₂H₆ gas is supplied is a flow rate lower thanthe lower limit, the resistivity lowering effect is insignificant.

FIG. 5 shows the dependence of resistivity on 5% B₂H₆ gas supply rate.The respective supply rates of WF₆ gas, Ar gas and H₂ gas are fixed atvalues specified in Table 1 showing conditions for tungsten film formingprocesses. As obvious from the graph shown in FIG. 5, resistivityincreases sharply as the supply rate of the 5% B₂H₆ gas increases beyond100 sccm (about 2% of the supply rate of all the gases), and increasesalso as the supply rate of the 5% B₂H₆ gas decreases below 10 sccm(about 0.2% of the supply rate of all the gases). Accordingly, it isdesirable that the supply rate of the 5% B₂H₆ gas is in the range of 10to 100 sccm.

The resistivity of the tungsten film can be reduced to some extent bysupplying the gases at a flow rate higher than a certain level relativeto the volume of the processing vessel 4. This point will be describedreferring to FIG. 6. FIG. 6 shows the dependence of resistivity on thetotal flow rate of all the gases in the tungsten film forming process.In FIG. 6, the total flow rate of all the gases and the ratio of thetotal flow rate of all the gases to the volume of the processing vessel4 are measured on the horizontal axis. The flow rate ratio between thegases is fixed at the value for the tungsten film forming process shownin Table 1 and the total flow rate of all the gases is varied.

As obvious from the graph shown in FIG. 6, resistivity decreasesgradually with the increase of the total flow rate of all the gases andremains substantially constant when the total flow rate of all thegasses increases beyond 1220 sccm. It is known from FIG. 6 that thetotal flow rate of all the gases in the tungsten film forming processmust be not lower than 1220 sccm, i.e., 100% or above of the volume ofthe processing vessel. Incidentally, the first embodiment (example 1)supplies the gases at 5050 sccm in the tungsten film forming process.

The resistivity of the tungsten film is greatly dependent on thethickness of the tungsten seed crystal grain layer. The thickness of thetungsten seed crystal grain layer must be about 50 nm or below to obtaina tungsten film having a resistivity of 12 μχcm or below. FIG. 7 showsthe dependence of resistivity on the thickness of the tungsten seedcrystal grain layer. It is obvious from FIG. 7 that resistivityincreases gradually with the increase of the thickness of the tungstenseed crystal grain layer, and the resistivity is greater than about 12μχcm when the thickness is greater than 50 nm. Therefore, it isdesirable that the thickness of the tungsten seed crystal grain layer is50 nm or below. However, it is difficult to form the tungsten film ifthe thickness of the tungsten seed crystal grain layer is excessivelythin and hence the lower limit of the thickness is about 10 nm.

Since the method according to the present invention uses the 5% B₂H₆ gasprepared by diluting B₂H₆ gas with H₂ gas as the bo-ron-containing gasinstead of a born-containing gas prepared by diluting B₂H₆ gas with N₂gas or Ar gas and used by the conventional method, the polymerization ofB₂H₆ gas does not occur in the gas container and B₂H₆ gas supply pipingand hence the clogging of piping with a solid formed by polymerizationcan be prevented. FIG. 8 is a graph showing the variation of B₂H₆concentration with time for a case where a nitrogen-diluted B₂H₆ gas isused (conventional method) and a case where a hydrogen-diluted B₂H₆ gasis used (present invention). As obvious from the graph shown in FIG. 8,the B₂H₆ concentration of the nitrogen-diluted B₂H₆ gas decreases withtime, which proves that B₂H₆ polymerizes. The B₂H₆ concentration of thehydrogen-diluted B₂H₆ gas employed in the method according to thepresent invention remains constant regardless of time, which proves thatB₂H₆ does not polymerize. It is considered that molecules of B₂H₆ becomeunstable when B₂H₆ gas is diluted with N₂ gas.

The tungsten film forming method in the first embodiment (example 1)carries out the boron-exposure process as shown in FIG. 2C between thetungsten seed crystal growing process and the tungsten film formingprocess. A tungsten film forming method in a second embodiment accordingto the present invention omits the boron-exposure process and has asimple pressure raising process instead of the boron-exposure process.The pressure raising process is carried out to purge residual gasesremaining in the processing vessel and to adjust pressure. FIGS. 9A, 9B,9C and 9D are views of assistance in explaining a preparatory process, aseed crystal growing process, a pressure raising process, and a tungstenfilm forming process, respectively, of the film forming method in thesecond embodiment, in which the processes excluding the process shown inFIG. 9C are the same as those shown in FIG. 2. In the pressure raisingprocess shown in FIG. 9C, the processing vessel 4 is evacuated to a basepressure, and then Ar gas, H₂ gas and N₂ gas are supplied at 2700, 1800and 900 sccm into the processing vessel 4 to increase the pressure inthe processing vessel 4 to the process pressure of 80 torr.

Process conditions for the tungsten film forming method in the secondembodiment excluding those for the pressure raising process are the sameas those for the tungsten film forming method in the first embodiment(example 1). Although not as effective as the first embodiment (example1), the second embodiment is capable of forming a tungsten film having aconsiderably low resistivity even though the boron-exposure process inthe first embodiment is omitted.

Comparative test of the conventional method and the methods in the firstembodiment (example 2, example 3) and the second embodiment of thepresent invention was conducted. Table 1 shows the respective flow ratesof gases for the tungsten film forming process, existence of the processof exposure to B₂H₆ gas and the resistivities of tungsten films formedby those methods. FIG. 10 is a graph showing sectional profiles of boronconcentration, FIG. 11 is a graph showing sectional profiles of chlorineconcentration, and FIG. 12 is a graph showing sectional profiles offluorine concentration.

TABLE 1 Boron- 5% exposure WF₆ B₂H₆ H₂ Ar process Resistivity Firstembodiment 20 20 750 3500 Included 8.82 (Example 2) First embodiment 2020 750 1000 Included 10.6 (Example 3) Second embodiment 20 20 750 1000Omitted 11.6 Conventional 20 — 750 1000 Omitted 14.5 method Flow rate:sccm

As obvious from Table 1, the resistivity of the tungsten film formed bythe conventional method is 14.5 μχcm, and the resistivity of thetungsten film formed by the method in the second embodiment omitting theboron-exposure process is 11.6 μχcm, which is lower than that of thetungsten film formed by the conventional method. The resistivities ofthe tungsten films formed by the method in the first embodiment are 10.6μχcm or below which are far lower than that of the tungsten film formedby the conventional method. The resistivity of the tungsten film formedby the method in Example 2 of the first embodiment supplying Ar gas atan increased flow rate is as low as 8.82 μχcm. Resistivity reducingeffect is significant when the total flow rate of all the gases is high.

As obvious from FIG. 10, boron concentrations in the tungsten filmsformed by the methods in Examples 2 and 3 of the first embodiment arehigh and those in the tungsten films formed by the method in the secondembodiment and the conventional methods are low. As obvious from FIG.11, chlorine concentration of the tungsten film formed by theconventional method is considerably high and undesirable, and those ofthe tungsten films formed by the methods in the first and the secondembodiment are considerably low and satisfactory.

As shown in FIG. 12, whereas the fluorine concentration of the tungstenfilm formed by the conventional method is high, those of the tungstenfilms formed by the methods in the first and the second embodiment arelow and satisfactory. It is inferred that both the chlorineconcentration and the fluorine concentration of the tungsten filmsformed by the methods according to the present invention are low becausethe mobility of electrons in the bo-ron-doped film is enhanced andresistance decreased due to the reduction of F and Cl, i.e., impurities,large crystal grains are formed and contact resistance at grainboundaries is reduced.

A tungsten film forming method in a third embodiment according to thepresent invention will be described hereinafter.

The tungsten film forming method in the third embodiment uses the vacuumprocessing system 2 shown in FIG. 1 used by the first and the secondembodiment.

The tungsten film forming method in the third embodiment is similar tothat in the first embodiment, matters connecting with the thirdembodiment and distinct from those connecting with the first embodimentwill mainly be described. A preparatory process and a seed crystalgrowing process shown in FIGS. 2A and 2B similar to those included inthe first embodiment are carried out.

Subsequently, a boron-exposure process is carried out as shown in FIG.2C. In the boron-exposure process, Ar gas, H₂ gas and 5% B₂H₆. gas aresupplied at 4000, 1800 and 100 sccm. Tungsten seed crystal grains 48 areexposed to boron. A boron-doped layer is formed by the B₂H₆ gas. Theboron-exposure process is continued, for example, for 28 s at a processtemperature of about 460° C.

After the completion of the boron-exposure process, a tungsten filmforming process is started.

First, WF₆ gas, Ar gas, H₂ gas and N₂ gas are supplied at 80, 900, 750and 100 sccm, respectively, to form a tungsten film. The supply of SiH₄gas and B₂H₆ gas is stopped. The same process pressure of 80 torr andthe same process temperature of 460° C. as those of the precedingprocess are used. Consequently, holes 42 (FIG. 2A) are filled up and, atthe same time, a wiring tungsten film 50 is formed as shown in FIG. 2D.The duration of the tungsten film forming process is, for example, about98 s. The overall thickness of the tungsten film 50 is 800 nm.

The boron-exposure process is carried out between the seed crystalgrowing process and a tungsten film forming process to form a boronlayer by exposing the tungsten seed crystal grains to, for example,diborane. Thus, the tungsten seed crystal grains grow large in thesucceeding process. Therefore, the tungsten film 50 has crystalstructure similar to bulk crystal structure and has a considerably lowresistivity.

Whereas the resistivity of a tungsten film formed by the conventionaltungsten film forming method not using B₂H₆ gas was about 12.2 μχcm(1500 Å), the resistivity of a tungsten film formed by the tungsten filmforming method according to the present invention was about 8.5 μχcm(1500 Å), which proved significant resistivity improvement.

FIGS. 13A and 13B are photographs taken by an electron microscope ofsections of holes filled up with a tungsten film by a conventionalmethod and a method according to the present invention, respectively. Asobvious from FIGS. 13A and 13B, tungsten crystal grains formed by themethod according to the present invention are greater than those formedby the conventional method and are similar to bulk crystal structure.

The method according to the present invention, similarly to theconventional method, had no problem and was satisfactory in ability tofill up the holes 42.

The boron-exposure process of the method according to the presentinvention supplies the hydrogen-diluted 5% B₂H₆ gas at 100 sccm in theboron-exposure process. Preferably, the flow rate of thehydrogen-diluted 5% B₂H₆ gas is about 50 sccm or above, i.e., about0.85% of the total flowrate of all the gases (−50×100/(4000+180+50)) orabove. The resistivity of the tungsten film is not very low when theflow rate of the 5% B₂H₆ gas is below about 50 sccm. This fact is thesame as that of the first embodiment illustrated in FIG. 4.

The method according to the present invention uses the hydrogen-diluted5% B₂H₆ gas. B₂H₆ contained in the hydro-gen-diluted 5% B₂H₆ gas,differing from that contained in a nitrogen-diluted or argon-dilutedB₂H₆ gas, does not polymerize in the B₂H₆ gas container and B₂H, gassupply piping and hence the clogging of the piping with a solid formedby polymerization can be prevented. FIG. 8 is a graph showing thevariation of B₂H₆ concentration with time for a nitrogen-diluted B₂H₆gas (conventional method) and a hydrogen-diluted B₂H₆ gas (presentinvention). As obvious from the graph shown in FIG. 8, the B₂H₆concentration of the nitrogen-diluted B₂H₆ gas decreases with time,which proves that B₂H, polymerizes. The B₂H₆ concentration of thehydrogen-diluted B₂H₆ gas employed in the method according to thepresent invention remains constant regardless of time, which proves thatB₂H₆ does not polymerize. It is considered that molecules of B₂H₆ becomeunstable when B₂H, gas is diluted with N₂ gas.

The flow rates of the gases, process temperatures and the processpressures mentioned in connection with the description of the preferredembodiments and shown in the table are only examples and not limitative.Although this embodiment uses the hydro-gen-diluted B₂H₆ gas having aB₂H₆ concentration of 5%, naturally, the limit values of flow ratechange according to the B₂H₅ concentration of the diluted B₂H₆ gas. Theboron-containing gas is not limited to diborane gas; theboron-containing gas may be any borane gas, such as tetraborane gas orpentaborane gas. The present invention is applicable to processingwafers of sizes other than that mentioned above. The workpiece is notlimited to the semiconductor wafer, but may be a glass substrate or anLCD substrate.

As is apparent from the foregoing description, the tungsten film formingmethod according to the present invention exercises the followingexcellent operations and effects.

According to the first feature of the present invention, the tungstenfilm is formed by exposing the tungsten seed crystal grains posed to theboron-containing gas, and then reduction is carried out by using theboron-containing gas in the tungsten film forming process. Therefore,tungsten crystal grains forming the tungsten film grow large and thetungsten film has a low resistivity.

According to the second feature of the present invention, the tungstenfilm is formed by growing tungsten seed crystal grains and carrying outreduction by using the boron-containing gas when forming the tungstenfilm. Therefore, tungsten crystal grains forming the tungsten film arelarge and the tungsten film has a considerably low resistivity.

When the hydrogen-diluted 5% B₂H₆ gas is used as a bo-ron-containinggas, the hydrogen-diluted 5% B₂H₆ gas is supplied at a flow rate equalto about 0.85% or above of the total flow rate of all the gases. Thus, atungsten film having a still lower resistivity can be formed. A tungstenfilm having a still lower resistivity can be formed when the tungstenseed crystal grain layer is formed in a thickness of 50 nm or below. Atungsten film having a still lower resistivity can be formed when thegases are supplied in the tungsten film forming process so that theamount of the gases supplied in one minute is about 100% or below of thevolume of the processing vessel.

The boron-exposure process is carried out between the seed crystalgrowing process and the tungsten film forming process to expose theworkpiece to the boron-containing gas. Thus, the tungsten seed crystalgrains grow large in the succeeding process. Therefore, the tungstenfilm having large tungsten crystal grains and a low resistivity can beformed.

What is claimed is:
 1. A method of forming a tungsten film on a surfaceof an object to be processed by a vacuum processing system, said methodcomprising the steps of: growing tungsten seed crystal grains on thesurface of the object to be processed in an atmosphere of a film forminggas containing tungsten atoms; exposing the object to be processed to anatmosphere of a boron-containing gas for a short time; and forming atungsten film by making the tungsten seed crystal grains grow in anatmosphere of a gas containing a film forming gas containing tungstenatoms, a hydrogen gas and a hydrogen-diluted boron-containing gas.
 2. Amethod of forming a tungsten film on a surface of an object to beprocessed by a vacuum processing system, said method comprising thesteps of: growing tungsten seed crystal grains on the surface of theobject to be processed in an atmosphere of a film forming gas containingtungsten atoms; and forming a tungsten film by making the tungsten seedcrystal grains grow in an atmosphere of a gas containing a film forminggas containing tungsten atoms, hydrogen gas, and a hydrogen-dilutedboron-containing gas.
 3. The method of forming a tungsten film on asurface of an object to be processed by a vacuum processing systemaccording to claim 1 or 2, wherein the boron-containing gas is ahydrogen-diluted 5% diborane gas, and the hydrogen-diluted 5% diboranegas is supplied at a flow rate being about 0.85% or above of a totalflow rate of all gases.
 4. The method of forming a tungsten film on asurface of an object to be processed by a vacuum processing systemaccording to claim 1 or 2, wherein the tungsten seed crystal grains areformed in a layer of a thickness of 50 nm or below.
 5. The method offorming a tungsten film on a surface of an object to be processed by avacuum processing system according to claim 1 or 2, wherein the gasesare supplied in the tungsten film forming process at a flow rate suchthat a volume of the gases upplied in one minute is 100% or above of avolume of processing vessel in the vacuum processing system.
 6. Themethod of forming a tungsten film on a surface of an object to beprocessed by a vacuum processing system according to claim 1 or 2,wherein the tungsten film forming step achieves both filling up holes inthe surface of the object to be processed and forming wiring linessimultaneously.
 7. A method of forming a tungsten film on a surface ofan object to be processed by a vacuum processing system, said methodcomprising the steps of: growing tungsten seed crystal grains on thesurface of the object to be processed in an atmosphere of a film forminggas containing tungsten atoms; exposing the object to be processed to anatmosphere of a hydrogen-diluted boron-containing gas for a short time;and forming a tungsten film by making the tungsten seed crystal grainsgrow in an atmosphere of a gas containing a film forming gas containingtungsten atoms.
 8. The method of forming a tungsten film on a surface ofan object to be processed by a vacuum processing system according toclaim 7, wherein the hydrogen-diluted boron-containing gas is ahydrogen-diluted 5% diborane gas, the flow rate of the hydrogen-dilute5% diborane gas is about 0.85% or above of the total flow rate of allthe gases.
 9. The method of forming a tungsten film on a surface of anobject to be processed by a vacuum processing system according to claim7, wherein the tungsten film forming step achieves both filling up holesin the surface of the object and forming wiring lines simultaneously.